An automatic transfer switch (ATS) is commonly used for power distribution systems having multiple utility feeds, inverters, generators, or uninterruptible power supplies (UPS). In the event of a power outage, an ATS will sense a primary input feed or power source loss and start or switch to an alternate power source, such as an emergency generator or battery back-up. When power is restored, the ATS automatically switches the load from the alternate power source back to the primary power source.
ATSs are often used when no downtime from a power outage is tolerated. For example, uninterruptible power supplies (UPSs) are often used to provide auxiliary back up power, such as a battery, to provide uninterrupted power for critical loads, such as computer systems and other data processing systems. UPSs are also used to help protect systems from lightning, power surges and sags, electrical line noise, utility outages, and wild voltage fluctuations.
However, typical UPSs are failure prone. An ATS enables using two or more power sources or UPSs together in a redundant fashion, so if one fails, or failure occurs from some other cause, the ATS will transfer the load to the other power source or UPS. This transfer is typically designed to be fast enough, such as through solid state relays, so that virtually all computer loads or telecommunications systems are unaffected. In addition, ATSs can provide an alternative to UPSs. For example, many hospital facilities with access to two utility grids are now replacing their failure prone UPSs with ATSs.
ATSs have found wide use. In the event of power failure, ATSs allow for uninterrupted operation of many electronic systems, such as home, business, computer, medical, and telecommunications systems. ATSs are typically used to protect Internet service providers, electronics, controls, network servers, imaging and audio systems, personal computers, modems, satellite systems, surveillance cameras, and telephones against damaging power surges and even lightning. Also, ATSs have been used to protect against the negative effects on circuitry from line noise, grid switches, power outage transients, and other electrical events.
Standard ATSs typically contain two input feeds. A primary input feed typically supplies power to all of the outlets in the ATS. If the primary input feed fails, typical ATSs transfer all of the outlets to a secondary input feed. The secondary input feed supplies power to all of the outlets until the primary input feed is restored. When the primary feed is restored, all of the outlets are transferred back to the primary feed.
Since the entire load is powered by one power source, the entire load must be transferred between power sources. Transferring an entire load may cause premature equipment wear, such as on relays or other parts used to transfer the load. Further, transfer of the entire load can generate large amounts of heat. Similarly, the voltage drop that occurs when switching a high-current load between power sources can be quite large. Typically, when power is restored, all outlets are immediately switched back to the primary power source, potentially causing an overload due to the high in-rush current.
Because under normal operating conditions one power feed supplies all of the outlets in an ATS, ATS systems typically use battery back up or generators as secondary feeds.
Typical ATSs are designed to operate under a particular operating voltage range. Manufacturers typically produce a variety of ATSs for a variety of operating conditions. However, if a customer buys an ATS with a particular operating range and later needs an ATS having a different operating range, the customer typically must purchase a new ATS. This can be very expensive and inconvenient for the customer. In addition, producing a variety of ATSs to meet the varied operating voltage ranges can result in higher manufacturing costs than if fewer ATSs were produced.
In transferring between sources of power in, for example, a transfer switch, one problem involves the suppression of (i) arcing across a relay gap as the relay opens or closes and (ii) associated wear and tear on the relay and possibly other components. Existing arc suppression circuitry, such as that described in U.S. Pat. No. 6,741,435 to Cleveland (“POWER CONTROLLER WITH DC ARC-SUPPRESSION RELAYS”), generally involves the use of an electromechanical relay providing for DC electricity to be controlled between a power source and an electrical load. When the DC power source experiences an interruption, the circuitry taught by the Cleveland patent utilizes a transistor to actively shunt current while the relay switches positions. The arc suppression circuit taught by the Cleveland patent is, however, not adapted for use in AC circuits.